Single-lock delay locked loop with cycle counter and method therefore

ABSTRACT

Once a delay locked loop has been locked to a clock signal, an omitted clock cycle is injected into the input of the delay locked loop without stopping the operation of the delay locked loop. The omitted cycle is later detected at an output of the delay locked loop, and the delay between the input and output is determined based on the time the omitted cycle requires to propagate through the delay locked loop. Once determined, the number of cycles of delay for the delay locked loop can be used in conjunction with an internal clock signal to launch data and/or data strobes from memory devices and memory controllers such that the proper phase alignment and clock cycle alignment is achieved.

TECHNICAL FIELD

The disclosure herein relates generally to delay locked loops (DLLs) andmore particularly to circuits and methods for determining a number ofclock cycles of delay in such delay lock loops and their use inmemories.

BACKGROUND

Delay locked loops are well known in the art for monitoring andadjusting delays of timing signals used with circuitry on integratedcircuits, where the delays through the circuitry can vary with thechanging conditions experienced by the integrated circuits. For example,higher temperatures can increase the delay through circuit components.Similarly, fluctuations in the voltage supply provided to an integratedcircuit can cause the delays through the circuit components tofluctuate, where higher voltages may reduce delays and lower voltagesmay increase delays. Such delay variations can also be affected byprocessing variations during the manufacture of the integrated circuits.

In order to provide precise timing for signals and account for thevariation in delay through the circuit components, delay locked loopstypically include replicas or dummy-versions of the circuit componentsin a feedback path through which a periodic signal such as a clocksignal is propagated. Delay lines included in the delay locked loopdelay the periodic signal for a variable amount of time in order toachieve a desired phase relationship between a signal used to timeoperations performed on the integrated circuit and the periodic signal.As frequencies of clock signals used on and with integrated circuitsincrease, thereby resulting in shorter periods for those clock signals,the total delay through a delay locked loop can be longer than the timefor a single period of the clock signal provided to the delay lockedloop. In other words, a first positive edge of the clock provided to theloop may not have exited the delay locked loop before the next positiveedge has entered the loop. In such systems, the number of clock cyclesof delay through the delay locked loop can be important in terms ofdetermining in which clock cycle certain events occur. Thus, while thedelay locked loop may provide desirable phase alignment for a timingevent with respect to a clock signal, it may also be necessary to ensurethe event occurs in the correct clock cycle.

Therefore, it is desirable to provide techniques for determining anumber of clock cycles of delay through a delay locked loop with minimalinterference with the operation of the delay locked loop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram corresponding to a delay locked loop andassociated circuitry in accordance with an exemplary embodiment;

FIG. 2 is a timing diagram associated with the circuitry of FIG. 1during the determination of a number of clock cycles of delay throughthe delay locked loop in accordance with an exemplary embodiment;

FIG. 3 is a flow chart corresponding to a method for operating a delaylocked loop in accordance with an exemplary embodiment; and

FIG. 4 is a flow chart corresponding to a method for operating a memorythat includes a delay locked loop in accordance with an exemplaryembodiment.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations.

For simplicity and clarity of illustration, the figures depict thegeneral structure and/or manner of construction of the variousembodiments. Descriptions and details of well-known features andtechniques may be omitted to avoid unnecessarily obscuring otherfeatures. Elements in the figures are not necessarily drawn to scale:the dimensions of some features may be exaggerated relative to otherelements to assist improve understanding of the example embodiments.

The terms “comprise,” “include,” “have” and any variations thereof areused synonymously to denote non-exclusive inclusion. The term“exemplary” is used in the sense of “example,” rather than “ideal.”

In the interest of conciseness, conventional techniques, structures, andprinciples known by those skilled in the art may not be describedherein, including, for example, memory operation protocols, constructionof delay lines used in delay locked loops, and basic principles ofmemory devices. During the course of this description, like numbers maybe used to identify like elements according to the different figuresthat illustrate the various exemplary embodiments.

For the sake of brevity, conventional techniques related to clocksignals, reading and writing memory, and other functional aspects ofcertain systems and subsystems (and the individual operating componentsthereof) may not be described in detail herein. Furthermore, theconnecting lines shown in the various figures contained herein areintended to represent exemplary functional relationships and/or physicalcouplings between the various elements. It should be noted that manyalternative or additional functional relationships or physicalconnections may be present in an embodiment of the subject matter.

Nonvolatile memory devices are often used in various applications inorder to store data that remains valid after power is no longer applied.Examples of such nonvolatile memory devices include resistive memorydevices such as magnetic random access memories (MRAMs) in which theresistance experienced by current flowing through the memory cell variesbased on the orientation of magnetic vectors within the memory cell andwhere such resistance variations are used to store data. Spin torquememory devices send a current through the magnetoresistive stack inorder to store information in a memory cell. Depending on the directionof the current, and assuming it has adequate magnitude, the free portionof the memory cell will be forced into a corresponding magnetic stateindicative of the information to be stored in the cell.

MRAMs that support the double data rate synchronous dynamic randomaccess memory (DDR SDRAM) operation protocols can be used to providenon-volatile memory storage in systems without requiring modificationsto, or at least without requiring extensive modifications to, existingmemory controllers. The DDR SDRAM protocol is a standardized protocolthat utilizes both the rising edge and the falling edge of each clockcycle to transfer data in a memory system. As the frequencies of theclock signals used with such data transfer protocols continue toincrease, the amount of time included in each cycle of the clock signalis reduced. For example, DDR SDRAM devices supporting a 200 MHz clockrate are available, where each clock cycle only includes 5 ns. When bothedges are used to transfer data, data is delivered every 2.5 ns.

In order to help ensure accurate data transmission in such high-speedsystems, a data strobe can be sent with the data where the transition(edge) of the data strobe at the receiving end of the interface can beused to help capture the data. Because the routing of the signal linesused to carry the data signals and the routing of the signal line usedto carry the data strobe are the same or at least very similar, the timeof flight associated with those signals is generally the same. In otherwords, the transition on the strobe signal will take the same amount oftime to travel from the output of the sending integrated circuit to theinput of the receiving integrated circuit as the data signals.

Data transfer in such high speed memories is further facilitated byproviding a desired level of alignment or synchronization betweentransitions on the data strobe signal and the clock signal used tosynchronize the memory system. For example, in some embodiments it isdesirable for the data strobe signal to transition in alignment witheither a rising or falling edge of the clock signal used to synchronizethe memory system.

Because the data strobe signal, as well as the data signals themselves,are generated on an integrated circuit such as a memory or a memorycontroller, there is internal delay that must be accounted for in orderto achieve the desired alignment between the signals external to thoseintegrated circuits. For example, an internal signal that causes thedata or data strobe to be launched is typically asserted well before theactual transition of the external clock edge with respect to which thedata or data strobe is to be synchronized or aligned. Launching the dataor data strobe ahead of the clock transition allows the data or datastrobe to move through the internal circuitry of the integrated circuit,where after propagating through the internal circuitry and the delay ofthat internal circuitry, the data or data strobe is driven at theoutputs of the integrated circuit in proper synchronization with theexternal clock signal.

As discussed above, the delay through the internal components ofintegrated circuits can vary based on changes in temperature, voltage,processing, etc. As is known in the art, those variations in delay ofthe internal components can be accounted for and adjusted for by using adelay locked loop to replicate the components or create dummy versionsof the components and monitor the changes in the delay through thosereplicated or dummy components. Thus, the delay locked loop can providethe needed adjustment to the time at which a signal launch occurs withinthe integrated circuit such that the phase of the signal output isproperly aligned with the clock signal at the edge of the integratedcircuit.

In many protocols, including the DDR SDRAM protocol, the delay lockedloop is given a certain number of clock cycles to achieve a “locked”state with respect to the clock signal. The locked state is achievedwhen a phase detector in the delay locked loop determines that thedesired phase relationship between the clock signal received by thedelay locked loop and a feedback block signal has been achieved. Asdiscussed below, the phase detector provides input to adjust delays inthe delay locked loop until the delay locked loop is locked.

In many systems, including memory systems that utilize the DDR SDRAMprotocol, the specific clock cycle within which certain events are tooccur is important in order to allow the accurate transmission of dataand control signals. In the DDR SDRAM protocol, the latency between thereceipt of a request perform certain operations and the transfer of dataassociated with those operations is controlled and can be programmedusing mode registers on the memory. While different protocols implementthe delay between receipt of a request and the transmission of theassociated data in different ways, a simple version of the concept is tostore a value in a register, where the value is representative of anumber of clock cycles between receipt of a request to transfer data andthe transference of that data. For example, the register can store aread latency value, where a first value stored in the register indicatesthat data should be output seven clock cycles after the request for thedata has been received, whereas in another instance a second value inthe register indicates that the data should be output eight clock cyclesafter the request.

As such, the internal signal that causes the data to be launched by thememory needs to occur not only in the proper phase alignment with theexternal clock signal, but also in a manner that takes into account howmany clock cycles of delay may exist between the circuit receiving thelaunch signal and the actual output at the edge of the memory. Forexample, if the multiplexers and output buffers in the output path ofthe data downstream from the launching circuitry have a total delay onthe order of three clock cycles, then the launch indication needs to beprovided at least three clock cycles ahead of time in order for the datato traverse that output circuitry and arrive at the outputs of thememory during the proper clock cycle.

As such, it is important to know the total number of clock cycles ofdelay that exist in a delay locked loop in order to know the properclock cycle within which to cause data or a strobe signal to belaunched. As discussed herein, such a determination is best made whilethe delay locked loop is in the locked state and the delay linesassociated with the delay locked loop are being exercised under thenormal loading conditions present during regular operation of the delaylocked loop. When the input signal to the delay locked loop is periodic,all of the cycles should be virtually identical. As such, it can bedifficult to measure the delay through the delay line, as the output ofthe delay line in the delay locked loop is also a periodic signal.

As described herein, injecting a skipped cycle into the periodic signalbeing provided to the delay locked loop can be used to provide adistinguishing signal within the normally periodic signal, where theskipped cycle can be detected at the output of the delay locked loop inorder to determine how many clock cycles the skipped cycle required totraverse the delay locked loop. Because the skipped cycle is only asingle cycle within the periodic signal, it has minimal impact on theloading characteristics associated with the delay lines included in thedelay locked loop. In other words, rather than stopping the delay lockedloop and sending a discrete signal through the delay locked loopcircuitry that can be detected at the output in order to measure thedelay through the delay locked loop, the delay locked loop maintains alocked state during the period of time associated with the skipped cyclemoving through the delay locked loop. As such, the loadingcharacteristics within the delay locked loop are not dramaticallychanged as is the case when the delay locked loop is stopped.

Moreover, because the delay locked loop has already achieved the lockedstate when the skipped cycle is injected, the time associated withinitially locking the delay locked loop and then detecting the totalnumber of clock cycles of delay through the delay locked loop isminimized. In protocols that provide for a limited time to lock thedelay locked loop, this is advantageous as the delay locked loop onlyneeds to be locked once. In other systems in which the number of cyclesis measured after the delay locked loop is first locked and then stoppedto allow for the measurement, the delay locked loop needs to bere-locked after determination of the total number of cycles through thedelay locked loop. Having to lock the delay locked loop multiple timescan result in the memory system exceeding the specified time availablefor delay locked loop start up.

FIG. 1 illustrates a block diagram of a delay locked loop and associatedcircuitry where a skip cycle can be used to determine the amount ofdelay through the delay locked loop. A clock signal 12 is received bythe circuitry of FIG. 1, where in some embodiments the clock signal 12corresponds to an external clock signal to which data and data strobealignment is desired. The clock signal 12 is provided to first delayblock 10, where in some embodiments, the first delay block 10 includesinput buffers or other clock buffering circuits that help distribute theclock signal 12 throughout the integrated circuit on which the circuitryof FIG. 1 is included. As discussed herein, the first delay block 10also includes circuitry that can be used to insert a skip cycle withinthe clock signal 12 based on a skip control signal 62 received by thefirst delay block 10.

As one of ordinary skill in the art appreciates, many circuits can beused to insert a skip cycle into the clock signal 12. In one embodiment,the first delay block 10 simply gates off the clock signal 12 at itsoutput, thereby holding the DLL input 14 low or high and eliminating thetransitions associated with the skip cycle. For example, an AND gate canbe used where one input is the clock signal 12 and the other input isskip control signal 62. When the skip control signal 62 is asserted low,the output of the AND gate will be held low and the high portion of acycle of the clock signal 12 is gated off. The first delay block 10 canbe a distributed block, where different portions of the first delayblock can exist on different portions of the integrated circuit. Forexample, an initial clock buffer associated with receiving the clocksignal 12 and providing it to numerous different internal locations onthe integrated circuit may be physically distant from the circuitry usedto inject the skip cycle.

The output of the first delay block 10, which is labeled DLL input 14,is provided to a coarse delay block 30. In some embodiments, includingthose illustrated in FIG. 1, the total adjustable delay through thedelay locked loop is separated into a coarse delay and a fine delay,where a coarse delay block allows for larger adjustments to the totaldelay than the fine delay block. Each of the coarse and fine delayblocks typically includes a delay line. As known by one of skill in theart, the delay lines used in delay locked loops are often series ofinverters or buffers that propagate the received signal through theseries of inverters or buffers while the individual delay through eachof the inverters or buffers can be controlled in order to vary theoverall delay through each of the delay lines. The delay through eachinverter or buffer can be controlled by a regulator, where the regulatormay vary the supply voltages applied to the inverter or buffer such thatthe inverter or buffer switches slightly faster or slightly slowerbecause of the change in supply voltage.

The coarse delay block 30 performs coarse delay adjustments on the DLLinput signal 14 based one or more signals received from the delaycontrol block 50. The delay control block 50 can cause the delay throughthe coarse delay block 30 to be increased or decreased, where theincrements of delay change in the coarse delay block 30 are typicallylarger than those of the fine delay block 40. Similarly, the fine delayblock 40, which receives the output of the coarse delay blocks 30, canbe controlled by the delay control block 50 in order to further refinethe total delay experienced by the DLL input 14 as it goes through thetotal delay line that includes coarse delay block 30 and fine delayblock 40. Fine delay block 40 can also receive a signal from duty cyclecontrol block 80 in order to adjust the duty cycle of the signal as itpropagates through the delay line. Such duty cycle adjustment can beused in order to ensure a particular duty cycle (e.g. amount of time theperiodic signal is high versus the amount of time the periodic signal islow in each cycle) is maintained for the periodic signal.

As illustrated in FIG. 1, the output of the fine delay block 40 isprovided to a read path block 90 as well as a first delay replica 72.The read path block 90 corresponds to the circuitry used to launch theoutput signal from the memory, memory controller, or other device onwhich the delay locked loop circuitry is included. For example, the readpath block 90 in a memory may correspond to the circuitry used to launchoutput data from the memory, where the read path can includemultiplexers used to select the data from a larger set of data andbuffers used to amplify the signals corresponding to the data such thatsufficient drive strength is achieved at the output of the memory.

The first delay replica 72 is circuitry intended to replicate the delayexperienced by the clock signal 12 as it moves through the first delayblock 10. In some embodiments, the first delay replica 72 includes thesame circuitry that is included in first delay block 10, whereas inother embodiments, the delay for the components in first delay block 10is emulated in some manner within the first delay replica 72 wherevariations in the delay through the components in first delay block 10are mirrored in the replica circuitry in first delay replica 72. Thus,the periodic signal entering first delay replica 72 should experiencethe same delay going through that block as it would going through firstdelay block 10 under the current conditions. If the delay through firstdelay block 10 changes based on, for example, a change in voltage ortemperature, delay through first delay replica 72 changes in the samemanner.

Similarly, the read path replica 74 mimics the delay through read pathblock 90. In some embodiments the same components can be included inread path replica 74 as are in read path block 90. In other embodiments,circuitry that mimics the delay through read path block 90 is includedin read path replica 74. Thus, under the same conditions, the delayexperienced by a signal moving through read path replica 74 should matchthe delay experienced by that signal moving through read path block 90.Notably, the ordering of the first delay replica 72 and read pathreplica 74 can be reversed with respect to the propagation of the signalgoing through the delay locked loop. Similarly, the coarse and finedelay blocks 30 and 40 can be reordered such that the signal movingthrough the delay locked loop goes through the fine delay block 40before the coarse delay block 30.

The output of the read path replica 74, which is labeled as the feedbackclock 76, is provided to a phase detector 20, where the phase detector20 also receives the output of the first delay block 10. The phasedetector 20 compares the phase of the signals it receives and providesinput to the delay control block 50 in order to cause the delay controlblock 50 to adjust the delay through the coarse delay block 30 and/orthe fine delay block 40 to achieve a desired phase relationship betweenthe feedback clock signal 76 and the DLL input signal 14.

Once the phase detector 20 determines that the phase of the feedbackclock signal 76 and the DLL input signal 14 are in proper alignment, thedelay locked loop is considered to be locked. In some embodiments, thedesired alignment for these two signals is such that the rising andfalling edges of the two signals occur at the same time with a smallmargin of phase offset allowed. Once this occurs, the phase detector 20signals loop monitor 60 that the delay locked loop is locked. The loopmonitor 60, which receives the feedback clock signal 76 in addition tothe signal indicating a locked condition from the phase detector 20,initiates a calibration mode in response to receipt of the lock signalfrom the phase detector 20.

During the calibration mode, the loop monitor 60 asserts skip controlsignal 62 to the first delay block 10 such that the first delay block 10omits a clock cycle in the DLL input signal 14. In some embodiments, theloop monitor 60 asserts the skip control signal 62 for time periodcorresponding to one clock cycle of the clock signal 12 such that thatskip control signal can be used to gate off or restrict transitionswithin the DLL input signal 14. Thus, when the skip control signal 62 isde-asserted, the first delay block 10 outputs a delayed version of theinput clock signal 12 to the input of the delay locked loop. When theskip control signal 62 is asserted, the first delay block 10 outputs anon-transitioning signal as the DLL input signal 14. The lack oftransitions within the DLL input signal 14 corresponds to an omittedclock cycle in the delayed version of the clock signal 12.

When the loop monitor 60 asserts the skip control signal 62, the loopmonitor 60 also starts keeping track of the number of clock cycles thattranspire after asserting the skip control signal 62. The omitted clocksignal injected into the DLL input 14 propagates through the coarsedelay block 30, the fine delay block 40, the first delay replica 72, andthe read path replica 74 before arriving at the loop monitor 60 as apart of the feedback clock signal 76. When the loop monitor 60 detectsthe omitted clock cycle in the feedback clock signal 76, it determinesthe number of clock cycles of the clock signal in the delay of the delaylocked loop based on the detection of the omitted clock signal. Thus, ifthe skip control signal 62 is asserted in clock cycle “1” and theomitted clock signal is detected in the feedback clock 76 in clock cycle“4”, the number of cycles of delay in the delay locked loop correspondsto three cycles. In some embodiments, the number of cycles of delay inthe delay locked loop determined is an integer number of clock cycles,whereas in other embodiments, fractional portions of clock cycles canalso be counted or otherwise detected.

In order to determine the number of clock cycles of delay in the delaylocked loop, the loop monitor 60 can include a counter 61, where thecounter starts counting when the skip control signal 62 is asserted andstops counting when the loop monitor 60 detects the omitted clock signalin the feedback clock signal 76. Loop monitor 60 can also include a skipdetector 63, where the skip detector 63 is able to detect the presenceof the omitted clock signal within the feedback clock 76. In someembodiments, the skip detector 63 includes a circuit that compares thecurrent cycle of the feedback clock signal 76 with the previous cycle ofthe feedback clock signal 76. For periodic signal, the current cycle offeedback clock signal 76 and the previous cycle of the feedback clocksignal 76 should be virtually the same. However, when the omitted clocksignal, which corresponds to a lack of transitions within the feedbackclock signal 76, is compared with the previous cycle of the feedbackclock signal 76, the lack of transitions corresponding to the omittedclock signal are readily detected and recognized as the omitted clocksignal.

Once the loop monitor 60 has detected the omitted clock signal anddetermined the number of clock cycles of delay within the delay lockedloop, the loop monitor 60 asserts a detection complete signal 64 andoutputs at least one signal representative of the number of cycles inthe delay of the delay locked loop, which is shown in FIG. 1 as numberof cycles signal 66. In one example embodiment, the number of cyclessignal 66 is a plurality of bits encoding the number of cycles of delaywithin the delay locked loop.

Control circuitry 80 which is coupled to the loop monitor 60 and theread path 90, receives the detection complete signal 64 and the numberof cycles of delay 66 and adjusts timing within the read path 90 toensure that signals are output by the delay path 90 in the desired clockcycle based in part on the number of cycles of delay through the delaylocked loop. For example, the control circuitry 80 can adjust when theoutput signal is initiated by the read path 90 in a manner such that theoutput signals output from a memory are in phase alignment with theexternal clock signal and are output during a selected cycle of theexternal clock signal. The selected cycle of the external clock signalcan be the particular clock cycle specified through the use of an accesstime register or a similar latency register or circuit.

Upon start-up, the delay locked loop circuit of FIG. 1 only needs to belocked once, and the lock is maintained while the loop monitor 60determines the number of clock cycles of delay through the delay lockedloop. By only inserting a single omitted clock cycle in the DLL inputsignal 14 provided to the delay locked loop, the loading on the coarsedelay block 30 and find delay block 40 is minimally impacted, therebygiving an accurate determination of the number of clock cycles of delaythrough the delay locked loop. This is in contrast to prior art systemsin which the delay locked loop was stopped and the delay through theloop measured using signals propagated through the loop in a manner thataltered the loading, and therefore the delay characteristics, of thecomponents within the loop.

FIG. 2 provides a timing diagram that illustrates the timingrelationships of certain signals associated with the circuitry ofFIG. 1. Clock signal 12 is shown at the top of FIG. 2 to be a periodicsignal that transitions regularly between the high and low states andhas a duty cycle that is approximately 50/50. While the clock signal 12in FIG. 2 is depicted as a single ended clock signal, and otherembodiments, the clock signal may be a differential clock signal. Inoperation, the clock signal 12, which may correspond to a clock signalto which data and/or data strobe transmission is to be synchronized, isprovided to the delay locked loop via the first delay block 10 as DLLinput signal 14. Once the delay locked loop locks on to the clock signal12, the loop monitor 60 asserts skip control signal 62 at edge 101. Withthe skip control signal 62 asserted, the first delay block 10 omits theomitted clock cycle in the DLL input signal 14. The omitted clock cycleis shown between time points 102 and 103.

The omitted clock signal injected into the DLL input signal 14propagates through the delay locked loop and reappears in the feedbackclock signal 76 between time points 104 and 105. Shortly thereafter, theloop monitor 60 asserts the detection complete signal 64 at 106 andoutputs a value (VALUE) on the number of cycles signal(s) 66. In theexample shown in FIG. 2, the delay through the delay locked loopexperienced by the omitted clock signal is approximately four clockcycles. The single omitted clock cycle has a minimum impact on thenumber of transitions in the DLL input signal 14.

Notably, in other embodiments, more than one omitted clock cycle can beincluded in the DLL input signal 14, where multiple omitted clock cyclesmay improve detectability by the loop monitor. In the embodiment shownin FIG. 2 the omitted clock signal corresponds to the DLL input signal14 being held low. In other embodiments, inserting an omitted clockcycle corresponds to holding the DLL input signal 14 high. In eithercase, the omitted clock signal is represented by a lack of transitions,thereby providing easy detectability by the loop monitor. In yet otherembodiments, rather than completely omitting one or more clock cycles,one or more clock cycles can be manipulated in order to enable detectionof the manipulated clock cycle(s) at the output of the delay lockedloop. For example, the amplitude of the high and low transitions withinthe manipulated clock cycle can be reduced such that the manipulatedclock cycle can still be detected by the loop monitor, while such amanipulated clock cycle may have an even lesser impact on the loadingassociated with the delay locked loop circuitry.

FIGS. 3 and 4 are flow charts that illustrate exemplary embodiments oraspects of embodiments of a method for operating a delay locked loop anda method for operating a memory that includes a delay locked loop. Inone example, the memory is a magnetic memory that supports a DDR SDRAMprotocol. The operations included in the flow charts may represent onlya portion of the overall process used in operating the delay locked loopand memory. For illustrative purposes, the following description of themethods in FIGS. 3 and 4 may refer to elements mentioned above inconnection with FIG. 1. It should be appreciated that methods mayinclude any number of additional or alternative tasks, the tasks shownin FIGS. 3 and 4 need not be performed in the illustrated order, and themethods may be incorporated into a more comprehensive procedure orprocess having additional functionality not described in detail herein.Moreover, one or more of the tasks shown in FIGS. 3 and 4 could beomitted from an embodiment as long as the intended overall functionalityremains intact.

FIG. 3 illustrates a flow chart of a method for operating a delay lockedloop, where the delay locked loop has a first end and a second end andwhere there is a delay experienced by a signal propagating through thedelay locked loop. At 310, a clock signal is provided to the first endof the delay locked loop where the clock signal includes a plurality ofcycles. As discussed above, the clock signal is a periodic signal and,in some embodiments, corresponds to the clock signal to which data, adata strobe, or some other signals should be synchronized. In someembodiments, the clock signal is a differential clock signal.

At 312, it is detected that the delay locked loop has locked to theclock signal. For example, once the phase detector within the delaylocked loop has determined that the adjustments that have been made tothe delay elements within the delay locked loop are such that thefeedback clock signal in the delay locked loop is properly aligned withthe clock signal being provided to the first end of the delay lockedloop, the delay locked loop is considered to be locked. At that point,the phase detector can signal that the locked condition has beenachieved.

Once it is determined that the delay locked loop has locked to the clocksignal, a cycle of the plurality of cycles included in the clock signalis omitted at 314. Thus, the clock signal as provided to the delaylocked loop is modified such that an omitted cycle is included withinthe signal as it is provided to the first end of the delay locked loop.In some embodiments, the cycle is omitted by gating off the clock signalfrom the first end of the delay locked loop for a time corresponding tothe omitted cycle. In other embodiments, a logic gate can be used tocombine a skip control signal indicating that a cycle should be omittedwith the clock signal to produce the signal provided to the first end ofthe delay locked loop such that the signal provided includes an omittedcycle. In some embodiments, omitting the omitted cycle includes holdingan input at the first end of the delay locked loop low during a cycle ofthe clock signal corresponding to the omitted cycle. In otherembodiments, omitting the omitted cycle includes holding the input atthe first end of the delay locked loop high during a cycle of the clocksignal corresponding to the omitted cycle.

As discussed above, once provided to the delay locked loop, the omittedcycle propagates through the delay locked loop and eventually appears inthe feedback clock signal at the second end of the delay locked loop. At316, the omitted cycle is detected at the second end of the delay lockedloop. In some embodiments, the detection of the omitted cycle isaccomplished by comparing the current cycle of the signal at the secondend of the delay locked loop with a previous cycle of the signal at thesecond end of the delay locked loop. In such a comparison, the absenceof transitions corresponding to the omitted cycle are detected such thatthe omitted cycle is recognized.

After the omitted cycle is detected at 316 a number of cycles of theclock signal in the delay through the delay locked loop is determined at318. The number of clock cycles in the delay corresponds to a number ofclock cycles that transpires while the omitted cycle propagates from thefirst end of the delay locked loop to the second end of the delay lockedloop. In one embodiment, determining the number of cycles at 318includes starting a counter when the omitted cycle is provided to thefirst end of the delay locked loop and then determining the number ofcycles based on the counter when the omitted cycle is detected at thesecond end of the delay locked loop. As noted above, in someembodiments, the number of cycles in the delay is an integer number ofclock cycles. In other embodiments, a fractional number of cycles can bedetermined.

Notably, while the number of cycles from a first end of the delay lockedloop to the second end of the delay locked loop is determined in theembodiment illustrated in FIG. 3, in other embodiments, the delaythrough a portion of the delay locked loop can be measured rather thanthe entire delay from end-to-end. For example, referring to FIG. 1, theloop monitor 60 could tap into the signal output from the fine delayblock 40 to detect the omitted cycle and measure the delay through onlythe coarse and fine delay blocks 30 and 40.

Once the number of cycles of delay through the delay locked loop hasbeen determined, an output signal can be generated that indicates thatthe number of cycles of delay has been determined. While the signalindicating that the number of clock cycles has been determined isoutput, at least one additional signal representative of the number ofcycles in the delay can also be output. Thus, once the number of cycleshas been determined, an indication that such a determination has beenmade can be signaled, and a value or set of signals that represents thenumber of cycles in the delay can be output for use by other componentsin the system.

FIG. 4 illustrates a flow chart of a method of operation of a memorythat includes a delay locked loop. The delay locked loop includes afirst end and a second end and has a delay associated with the time ittakes a signal to propagate from the first end of the delay locked loopto the second end. At 340, a clock signal is provided to the first endof the delay locked loop. As discussed above, the clock signal is aperiodic signal that includes a plurality of cycles.

At 342 it is detected that the delay locked loop has locked to the clocksignal. When the delay locked loop is locked the delay locked loopgenerates an internal clock signal to provide phase alignment for dataoutput from the memory. Such an internal clock signal can be used byother circuitry downstream from the delay locked loop in order tocontrol the timing of the signals that launch data and or a data strobefrom the memory such that timing of those signals is aligned with thesettings for the memory.

Once the delay locked loop has locked to the clock signal, a cycle isomitted in the clock signal as that signal is provided to the first endof the delay locked loop at 344. As discussed above, omission of a cyclecan be accomplished by holding the input at the first end of the delaylocked loop in the high or low state during the cycle to be omitted. Inembodiments involving a differential clock signal, the differentialinput is maintained in a state where no transitions occur during theomitted cycle. In some embodiments, when the omitted cycle is injectedinto the input clock corresponding to the delay locked loop, a counteris started at 346 in order to eventually provide a count of the numberof cycles of the clock signal present in the delay through the delaylocked loop.

At 348, the omitted cycle is detected at the second end of the delaylocked loop. Based on the detection of the omitted cycle at the secondend of the delay locked loop, the number of cycles of the clock signalincluded in the delay of the delay locked loop is determined at 350. Inembodiments in which a counter is used to count the number of cycles asthe omitted clock cycle propagates through the delay locked loop, thenumber of clock cycles is determined based on the value of the counterwhen the omitted cycle is detected.

At 352, the launch of data and/or a data strobe from the memory isinitiated based on an internal clock signal generated by the DLL and thenumber of cycles in the delay of the DLL as determined at 350. In someembodiments, initiating the launch of data and/or the data strobeincludes initiating the launch based on the internal clock signal suchthat data output from the memory is phase aligned with an external clocksignal received by the memory. In some embodiments initiating the launchof data includes initiating the launch such that the data outputcorresponds to a selected clock cycle of the external clock signal,where the selected clock cycle can be determined based on a value storedin a register on the memory. Such a value stored in a register may be alatency value stored in the register, where the latency value isrepresentative of a number of clock cycles of an external clock signalto transpire between the receipt of a read operation code or command bythe memory and the output of data corresponding to the read operationcode or command by the memory. For example, while the internal clocksignal generated by the delay locked loop may provide the needed phasealignment for the signal used to launch the data, the number of clockcycles of delay through the delay locked loop may be needed to ensurethat the data appears at the outputs of the memory during the correctclock cycle.

As the frequency of clock signals provided to memory devices and memorycontrollers within memory systems continues to increase, maintainingaccuracy of a determination as to how many clock cycles of delay arepresent in a delay lock loop can become more difficult. As such, havingless of an impact on the delay lock loop while such a determination isbeing made becomes increasingly important. By using an omitted clockcycle that propagates through the delay lock loop to determine thenumber of clock cycles of delay that exist within the components of thedelay locked loop, such a determination can be made without stopping thedelay locked loop and with minimal influence on the delay through thecomponents included within the delay locked loop. In embodiments such asmemory devices or memory controllers that include such a delay lockedloop, only needing to lock the delay locked loop once can beadvantageous in meeting start-up timing parameters associated with amaximum time for locking the delay locked loop.

While exemplary embodiments have been presented above, it should beappreciated that many variations exist. Furthermore, while thedescription uses memory devices and memory controllers as exampleintegrated circuits that can include delay locked loops benefiting fromthe teachings herein, the teachings may be applied to other integratedcircuits, devices, or systems having different architectures in whichthe same concepts can be applied. Moreover, while the embodiments abovediscuss launching data and data strobe signals from memory devices andmemory controllers, the same concepts can be applied to launching othersignals, such as, for example, address or control signals associatedwith data access operations.

The particular embodiments disclosed above are illustrative only andshould not be taken as limitations, as the embodiments may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein.Accordingly, the foregoing description is not intended to limit thedisclosure to the particular form set forth, but on the contrary, isintended to cover such alternatives, modifications and equivalents asmay be included within the spirit and scope of the inventions as definedby the appended claims so that those skilled in the art shouldunderstand that they can make various changes, substitutions andalterations without departing from the spirit and scope of theinventions in their broadest form.

What is claimed is:
 1. A method for operating a delay locked loop havinga first end and a second end, comprising: providing a clock signal tothe first end of the delay locked loop, wherein the clock signal is aperiodic signal that includes a plurality of cycles; detecting that thedelay locked loop has locked to the clock signal, wherein the delaylocked loop has a delay between the first end and the second end; afterdetecting that the delay locked loop has locked, omitting an omittedcycle of the plurality of cycles of the clock signal as the clock signalis provided to the delay locked loop at the first end; detecting, at thesecond end of the delay locked loop, the omitted cycle of the pluralityof cycles; and determining a number of cycles of the clock signal in thedelay of the delay locked loop, wherein the number of clock cycles inthe delay corresponds to a number of clock cycles that transpired whilethe omitted cycle of the plurality of cycles propagates from the firstend of the delay locked loop to the second end of the delay locked loop.2. The method of claim 1, wherein determining the number of cycles ofthe clock signal in the delay further comprises: starting a counter whenthe omitted cycle of the plurality of cycles is provided to the firstend of the delay locked loop; and determining the number of cycles ofthe clock signal in the delay based on the counter when the omittedcycle is detected at the second end of the delay locked loop.
 3. Themethod of claim 1, wherein omitting the omitted cycle of the pluralityof cycles further comprises gating off the clock signal from the firstend of the delay locked loop for a time corresponding to the omittedcycle.
 4. The method of claim 1 wherein determining the number of cyclesof the clock signal in the delay further comprises determining aninteger number of clock cycles in the delay.
 5. The method of claim 1wherein detecting the omitted cycle of the plurality of cycles furthercomprises comparing a signal at the second end of the delay locked loopduring a current cycle with the signal at the second end of the delaylocked loop during a previous cycle.
 6. The method of claim 1, furthercomprising: after determining the number of cycles of the clock signalin the delay, outputting a signal indicating that the number of cycleshas been determined; and while outputting the signal indicating that thenumber of cycles has been determined, outputting at least one additionalsignal representative of the number of cycles in the delay.
 7. Themethod of claim 1, wherein the clock signal is a differential clocksignal.
 8. The method of claim 1, wherein omitting the omitted cyclefurther comprises holding an input at the first end of the delay lockedloop low during a cycle of the clock signal corresponding to the omittedcycle.
 9. A method of operation of a memory that includes a delay lockedloop having a first end and a second end, the method comprising:providing a clock signal to the first end of the delay locked loop,wherein the clock signal is a periodic signal that includes a pluralityof cycles; detecting that the delay locked loop has locked to the clocksignal, wherein the delay locked loop has a delay between the first endand the second end, wherein when locked, the delay locked loop generatesan internal clock signal to provide phase alignment for data output fromthe memory; after detecting that the delay locked loop has locked,omitting an omitted cycle of the plurality of cycles of the clock signalas the clock signal is provided to the delay locked loop at the firstend; detecting, at the second end of the delay locked loop, the omittedcycle of the plurality of cycles; determining a number of cycles of theclock signal in the delay of the delay locked loop, wherein the numberof clock cycles in the delay corresponds to a number of clock cyclesthat transpired while the omitted cycle of the plurality of cyclespropagates from the first end of the delay locked loop to the second endof the delay locked loop; and initiating a launch of data from thememory based on the internal clock signal and the number of cycles ofthe clock signal in the delay of the delay locked loop.
 10. The methodof operation of the memory of claim 9, wherein initiating the launch ofdata further comprises initiating the launch of data based on theinternal clock signal such that data output from the memory is phasealigned with an external clock signal.
 11. The method of operation ofthe memory of claim 10, wherein initiating the launch of data furthercomprises initiating the launch of data such that the data output fromthe memory corresponds to a selected clock cycle of the external clocksignal.
 12. The method of operation of the memory of claim 11, whereinthe selected clock cycle is determined based on a value stored in aregister on the memory.
 13. The method of claim 12, wherein the selectedclock cycle is determined based on a latency value stored in theregister on the memory, wherein the latency value is representative of anumber of clock cycles of the external clock signal to transpire betweenthe receipt of a read operation code by the memory and the output ofdata corresponding to the read operation code by the memory.
 14. Themethod of claim 9, wherein determining the number of cycles of the clocksignal in the delay further comprises: starting a counter when theomitted cycle of the plurality of cycles is provided to the first end ofthe delay locked loop; and determining the number of cycles of the clocksignal in the delay based on the counter when the omitted cycle isdetected at the second end of the delay locked loop.
 15. The method ofclaim 9, wherein omitting the omitted cycle of the plurality of cyclesfurther comprises gating off the clock signal from the first end of thedelay locked loop for a time corresponding to the omitted cycle.
 16. Themethod of claim 9 wherein determining the number of cycles of the clocksignal in the delay further comprises determining an integer number ofclock cycles in the delay.
 17. An apparatus, comprising: a delay lockedloop that receives an input at a first end and generates a feedbackclock signal at a second end based on the input, wherein the delaylocked loop has a delay between the first end and the second end; afirst delay block coupled to the delay locked loop, wherein the firstdelay block receives a skip control signal and a clock signal, whereinthe clock signal is a periodic signal having a plurality of cycles,wherein the first delay block: outputs a delayed input clock signal tothe input of the delay locked loop when the skip control signal isde-asserted; and outputs a non-transitioning signal to the delay lockedloop when the skip control signal is asserted, wherein thenon-transitioning signal corresponds to an omitted clock cycle of thedelayed input clock signal; a loop monitor coupled to the delay lockedloop and the first delay block, wherein the loop monitor receives thefeedback clock signal and outputs the skip control signal, whereinduring a calibration mode, the loop monitor: asserts the skip controlsignal for a time period corresponding to a clock cycle of the clocksignal; and determines a number of cycles of the clock signal in thedelay of the delay locked loop based on detection of the omitted clockcycle in the feedback clock signal.
 18. The apparatus of claim 17,wherein, after the loop monitor determines the number of cycles of theclock signal, the loop monitor outputs a signal indicating detection iscomplete and at least one signal representative of the number of cyclesin the delay of the delay locked loop.
 19. The apparatus of claim 18,further comprising: control circuitry coupled to the loop monitor,wherein the control circuitry initiates an output signal from theapparatus based on an internal clock signal generated by the delaylocked loop and the signal representative of the number of cycles in thedelay.
 20. The apparatus of claim 19, wherein the apparatus is a memorythat receives an external clock signal, wherein the control circuitryinitiates the output signal such that the output signal is output fromthe memory in phase alignment with the external clock signal during aselected cycle of the external clock signal.